The present invention solves problems in inspecting transparent membranes at various stages in the manufacture of integrated circuits on silicon wafers by a microlithography process. However, the invention is applicable to inspection of other thin transparent membranes.
The microlithography technique is similar to photography except that glass coated with etched chrome, called a photomask, serves as the negative from which the integrated circuit is printed. The negative or photomask is formed from a photomask blank comprising a glass plate covered on one surface with a layer of chromium, which is in turn covered with a photo-resist membrane. The resist of the blank is exposed to a focused geometric light pattern, creating a photochemical change in the exposed area. The exposed blank is then subjected to a developing solution that removes exposed resist, and the blank is then exposed to an acid solution which removes the underlying chromium in the regions where the resist has been removed, etching the geometric pattern into the chromium. To protect the developed photomask from dust, the photomask is often packaged as a unit with a membrane or "pellicle" of transparent material, such as nitrocellulose or a PEHB membrane, overlying the etched surface and spaced therefrom by about 3-6 mm.
The pellicle-covered photomask is then used for projecting the geometric pattern onto a silicon wafer that is coated with a thin layer or membrane of photo-resist. By a multi-step process, the exposure pattern is developed into an integrated circuit of the precise projected geometric pattern.
The membrane that is used as the pellicle, the resist layer of the photomask blank and the resist layer covering the silicon wafer are all very thin, the pellicle typically being between about 0.8 and about 3 .mu.m. thick and the resist layers typically being between about 0.5 .mu.m. and about 2 .mu.m. thick. The microlithographic process requires high precision as even the smallest defects can result in an unsuitable product. Defects in either the resist or the pellicle may take the form of very tiny holes, wrinkles, uneven thicknesses and inclusion of particles, including transparent inclusion particles.
Because of the thinness of the resist and pellicle membranes, defects are often very difficult to detect. Defects in pellicles are even difficult to detect under a microscope because the defects do not produce any significant intensity or color change. Presently, in addition to microscopic examination, pellicles are inspected under monochromatic light with an unaided eye. However, this technique does not enable the observer to see low contrast defects having the size of a few hundredths of a millimeter. Furthermore, this is an additional step requiring additional technician time and an additional piece of equipment which occupies bench space.
It would be desirable to have a one-step process for inspecting thin membranes, such as pellicles for photomasks and resist layers.